Transmission and reception of frequency modulated waves



H. NYQUIST 2,188,758

TRANSMISSIQN AND RECEPTION OF FREQUENCY MODULATED WAVES Jan. 30, 1940.

2 Sheets-Sheet 1 Filed Aug. 24, 1938 '25 FIG. 3

AMPLITUDE DEMODULATOR I I I P 'INVENTOR H. NYQU/ST BY g ATTORNEY Jan. 30, 1940.

TRANSMISSION AND RECEPTION OF FREQUENCY MODULATED WAVES H. NYQUIST F-iled vAug. '24, 1958 BEE l l l l- Sheets-Shet 2 5/ SQZi/YAR/STOR I I I AAAA AMPLITUDE AMPLITUDE M00. /53 5 05M BRIT AMP

BEE

lNVENTOR H. N VQU/S T {Al-M A T TORNE V Patented Jan. 30, 1940 PATENT Fries TRANSMISSION AND RECEPTION OF FRE- QUENCY MODULATED WAVES Harry Nyquist, Millburn, N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York- Application August 24, 1938, Serial No. 226,444

9 Claims.

This invention relates to transmission and reception of frequency modulated waves.

An object of the invention is to simplify the apparatus required in frequency modulated wave systems.

Another object of and demodulation.

Demodulation of frequencymodulated waves 1 is ordinarily effected by converting the frequency modulated waves to waves having an amplitude modulation which may be readily demodulated by amplitude demodulators of the usual types. The conversion from frequency modulated waves to waves having anamplitude modulation is accomplished by subjecting the frequency modulated Waves to the action of a frequency selective path. The output current from such a path will vary in amplitude as the input electromotive force applied to the path varies in frequency.

It is customary in frequency modulation systerms to transmit carrier waves which vary in I frequency in accordance with the modulating sigrials but do not vary in amplitude. However, in consequence of the superposition of extraneous fields in the transmitting medium and also some forms of fading, thefrequency modulated waves upon arrival at a remote radio receiving station may exhibit amplitude modulations which appear 'as distortions when the frequency modulated waves are converted to amplitude modulated but are independent of amplitude fluctuations s not involving frequency modulation,

'One type of frequency modulation system uti lizes the modulating waves or impulses to control the reactance of an element of anoscillator frequency control circuit. Ferromagnetic inductances may be so employed as thecontrol react ance of the oscillator frequency control circuit in accordance with the method disclosed in Fig. 4

of the patent to Mertz 1,548,895, August 11, 1925. Such ferromagnetic inductances ordinarily tend to introduce a distortion because of the nonlinear relationship existing between the amplitude' of the modulating or controlling current and the resulting inductance of the ferromagnethe invention is to reduce the distortion incident to frequency modulation received frequency modulated waves.

tic element. Ideally, the inductance element should have such a characteristic that the resonance frequency of the oscillation circuit of which it forms a part will vary linearly with the amplitude of the modulating current. An object of the invention is to enable the ferromagnetic inductance element of such a frequency modwlating system to effectively present suchan ideal characteristic.' I I 1 In accordance with this invention-both fre quency modulation and demodulation of a frequency modulated Wave or either may be effected by varying the natural resonance frequency of a tuned circuit. Preferably this is accomplished by including in the tuned circuita variable reactance as, for example, an inductance having a ferromagnetic core similar to that disclosed in the patent to Mertz No. 1,548,895 to which refer once has been made. There maybe associated with the magnetic core a control winding sup- "plied with currents which by their magnetizing effect upon the core change the reactance of the inductance in well-known manner and thus vary the resonance frequency of the timed circuit.

Modulating systems in accordance with'this invention utilize the speech or other modulating waves or impulses as the control current for the variable inductance of the tuned circuit. Accordingly, if an electric wave generator be provided with a frequency-determining tuned circuit with such a magnetic control there will be produced modulated waves, the carrier frequency of which varies with the instantaneous variations in' amplitude of themodulating or control waves or impulses. v

Demodulating systems in accordance with this" invention utilize a similar variable tuned circuit which is so controlled by locally produced control currents that its natural resonance frequency tends to follow the frequency of the incoming or Any de parture from frequency agreement enables the incoming waves to set up in the resonance circuit an electromotive force which is applied to anassociated detector. The resulting detected current I 5 is then superimposed upon the original control current which thereupon alters the tuning of the tuned circuit to loring 'it into harmony with the frequency of the incoming waves. Under. these; conditions the control currents correspond with the original modulated control currents at the remote transmitter and may, accordingly, be utilized to yield 'speech'or other modulating signals by which the frequency modulation at the trans mitter station is effected. .9

If the natural frequency of a tuned circuit including a magnetically, controlled reactance does not vary in linear fashion with respect to the magnitude of the reactance or if the magnitude of the reactance does not vary in linear fashion with respect to the amplitude of the control cucrents the frequency of the tuned circuit will not bear a linear relation to the amplitude of the control current distortion will ensue. This may be effectively eliminated by a counter-distortion in which a similar resonant circuit or a device having a complementary resonant distorting characteristic is employed. In an alternative device the distortion may he effectively eliminated by a reverse feedback operation involving the principle disclosedin the U. S. PatentNo. 2,102; 671 to H. S. Black.

The nature of the invention will, be apparent from the followingdetailed description taken in connection with the appended and'the accompanying drawings in which: I

Fig. 1 illustrates a frequency modulation transmitter; 1

Fig. 2, a receiver for receiving frequency inoculated waves and deriving the modulated signals therefrom independently of the amplitude of received waves;

Fig. 3, a modified form of the receiver of Fig. 2; v

Fig. 4, a modification of the circuit of Fig. 3 in which'provision is made to reduce distortion occurring in the demodulating process;

Fig. 5, a receiving circuit for frequency modulated waves having alternative means for reducing distortion;

Fig. 6, a frequency modulated wave transmitter including means for reducing distortion which occurs in the modulating process;

Fig. '7, a modification of the-circuit of Fig. 6.

comprising an alternative arrangement for reducing distortion; and

Fig. 8,.a receiving system for frequency modu lated waves in which a local oscillator is so controlled as to tend to remainin frequency agreement and inphase quadrature with the incoming waves. 7

Referring to Fig. i, there is illustrated a vacuum tube oscillation generator comprising a tube tromotive force of the source l5 and by the magnitude of the resistance is in series with winding M. It is, therefore, possible to subject the magnetic core to any desired magnetomotive force to predetermine the normal inductance of the winding l3 as will be readily understood. Accordingly, during non-modulating intervals the oscillation source will produce oscillations of the frequency determined by the tuned circuit l l and will transmit them by Way of band-pass filter; l1 and power amplifier E8 to the transmitting antenna circuit ill. A microphone or other signal transmitter 29 is associated with an electric current source M in a series circuit coupling by means of the transformer 22 to the magnetizing current circuit of the control winding Hi. :Whenever, therefore, sound waves are impressed upon microphone 26 an electromotive force will be in-' duced in the secondary winding of transformer 22 to alternately augment and decrease the magnetizing current through the control winding 14.

The varying magnetomotive force which ensues changes the magnetization of the magnetic core and the magnitude of the'eifectiveinductance of the element 13. circuit H and, therefore, of the oscillations generated by the vacuum tube oscillator and trans-' y mitted from the antenna luwill rise and fall as the speech current varies the carrier frequency rising with one polarity of speech current and falling with the opposite polarity. In this mannor, the system of Fig. 1 serves to produce and radiate oscillations which are modulated as to,

ing from non-linearity,- the bandrpass filter lFl is provided. In general, the filter will have its mid-frequency at about-the frequency of the It follows that the tuning of unmodulatedcarrier wave and will readily trans? mit all frequencies at either side of the central frequency. over a range severalztimes that of the essential audio frequencies employed in ordinary speech. The power amplifier ill may be employed to-augment the energy within the desired: band for radiation. v t

Fig. 2 illustrates a receiving-system for use in receivingfreqllency modulated wavessuchas are transmitted fromzthe system of Fig, ,1. The antenna circuit ZS-Which, like the transmitting an-. tenna circuit i9; is preferably broadlyztuned. so I as to accept a very wide range of oscillations, without substantial" discrimination, and is coupled to band-pass filter 2A!- which; may be of the same type as the transmitting filter H. The high frequency energy received and selected by. the filter 24 is supplied'to two parallel paths one of which includesthe series resistances 2.5 and the primary Winding-of transformer Elk-the other of which includesthe primary windingof transformer 2l. shunted across i the first. path at;

points electrically adjacent the primary winding of transformer Zfi-is a circuit consisting ofan'f inductance element 28 and a capacity 29 in series therewith, the circuit being normallytuned to the unmodulated carrier frequency. Inductance 28;. resembles'the inductance E3 of the transmitter; in that it is associated with a similar ferromag netic core and a control winding 38. theznorma-lf current of which isdetermined by a source 3.! of

Associated with the sec-v ondary Winding of transformer 25. are the input.

circuits of two vacuum tube 'detectorstll and 331: Which are so connected that. an electromotive magnetizing current.

force in the primary winding of transformer "22,6;v

which induces a positive potential on the grid,

in the magnetizing path-of winding 35 in series with the primary winding of output transformer. i

36. The secondary winding of transformer 2'! is connected in acommonportion ofs'the input cir.-" cuits of devices 32 and 33 in such 'manner that. f an electromotive force in the primaryv windingv acting; in such direction as to make the grid of:

one tube positive, tendssimultaneously toimpress' 1 Y transformer 21 will cause equal electromotive forces of the same polarity to be applied to the input circuits oftubes 32 and 33. Assuming for the moment that the original space currents of the tubes 32 and 33 have been made equal so that without source 3| there is no potential difference between the outer extremities of resistances 34 and 35in consequence of these space currents, the similar changes which the space 2 currents undergo as a result of electromotive forces from transformer 21 will leave the electrical situation undisturbed. Since the tuned circuit 28, 29 is normally resonant at the unmodulated carrier wave frequency it will short-circuit the primary winding of transformer 26 for that frequency to prevent any effect upon detectors 32 and 33 from the other parallel path. Consequently, the receipt of oscillations of the normal carrier frequency regardless of their amplitude 3 which may be small or large, constant or variable, will not affect the detector output circuit. It will be apparent that the magnitude of the source 3| is determined solely by the criterion that the normal magnetizing current should maintain inductance 28 at such a magnitude that the circuit 28, 29 will be resonant to the normal carrier frequency. I

Whenever the carrier frequency of the incoming waves either rises or falls in consequence of modu- {o lation by speech currents at the transmitting station the operation of the circuit of Fig. 2 will change for the reason that the circuit 28, 29 no longer serves as a short circuit for the primary winding of transformer 26. Under these condi- 5:-tions oscillations induced in the secondary windings of transformer 26'will aid the oscillations induced in the secondarywinding of transformer 21in the input circuit of one detector tube at the .same time opposing those in the other. Accordlingly, space current in one tube will rise and that in the other will simultaneously fall so-that the changes in space current will occasion differences in potential in the resistances 34 and 35 and these difierencesinpotential are additive with respect to theouter terminals of the resistances. Because ofthe detecting action of the tubes these output circuit variation potentials will correspond to the original modulating speech frequency currents. It follows that detected speech frequency currents will be supplied by the transformer iit to the telephone receiver or other indicating device 31. Atthe same time, the current in the magnetizing circuit of winding 30 will undergo variations in consequence of the superposed de- *tected speech frequency electromotive force which will at times aid the normal magnetizing current from source 3I- and at times oppose it. The winding 30 is preferably so arranged that the speech current impulses which ensue upon rise of value will tend-to increase the resonant frequency of the circuit 28, 29. If the circuit be properly adjusted. the circuit 28, 29 will be retuned to the increased carrier frequency and .its. tuning will v thus tendto, follow that of the. incoming. waves.

" cillations appearing in the primary winding of.

the received carrier frequency from its normal In its attempt to automatically hold the tuned circuit 28, 29 at the frequency of the incoming waves the current of the magnetizing circuit of the winding 30 will correspond effectively to that in the magnetizing circuit of thewinding I4 to the extent that it is a faithful copy of the original speech current. It will, therefore, be seen that in itsoperation to demodulate the frequency modulated waves, the circuit of Fig. 2- servesto producefrom the incoming frequency modulated waves a variable magnetizing current which tends to control the reactance 28 so as to short-circuit the primary winding of transformer 26 and thus restore the normal zero output or balance condition of the detectors. In this manner it produces a magnetizing current which is a replica with re- ,spect to frequency and wave form of the speech frequency magnetizing current in the circuit of Fig. 1.

The circuit of the magnetizing winding in Fig. 2 constitutes a species of negative feedback and for best results should he so arranged that there is ahigh degree of feed back. As an indication of the adjustment and design of this circuit it may be stated that if the feedback circuit is disconnected from the magnetizing winding and a current is supplied to the magnetizing winding by some auxiliary source and the magnetizing current is changed, the resulting change in the output current should preferably be of the order of 10 to 100 times as great. Under these conditions of feedback and with the'circuit connected as shown in Fig. 2, the circuit stabilizes itself 'to a new Value of output current for each new frequency and the output current is a faithful copy of the modulating current at the remote transmitter.

The receiving circuit of Fig. 3 resembles that of 1 Fig. 2 but has been simplified by the omission of one of the detector tubes and the parallel path leading to transformerZl. The resonant circuit 39, Ml, instead of being normally tuned to the mid-frequency of the transmitted band is made resonant near one extremity of the desired band either above or below the mid-frequency. Consequently, during the non-modulating interval when the normal carrier frequencywave is being received the circuit 39, it presents a relatively large impedance to the incoming wave and an effective electromotive force of the incoming carrier frequency is applied to the detector input circuit. The resulting space current is preferably such as to attain the normal magnetizing current in winding M which is required to hold inductance 39 at the desired normal magnitude. Assume that the normal'tuningof the path 39, (it is such that its resonance frequency lies at the upper extremity of the wave band to be received. If new, as the modulation begins, the incoming carrier frequency at the receiver rises so as to approach the normal resonance frequency of the path 39, 40, vtheshunting effect of the resonance circuit will increase and the effective voltage applied to the input circuit of the detector tube will fall. The resulting decrease in the current in the magnetiz'ing and space current path of the detector operates to so change the inductance of element 39 that the resonance frequency of the path 38, ll becomes still higher thus receding from the frequency of the incoming waves. In this manner, the detector circuit tends to maintain the instantaneous tuning of the path 39, All at afrequency above the simultaneously received incoming waves. The varying magnetizing current to which the detection of the incomingwaves gives r41 corresponding in general to the series resisreceiver 4112 as the reproduced speech signal.

-. modulations of the incoming waves.

reason it may be desirable in such single detector rise'and which, in turn, controls the tuning of the path 39; Ml corresponds'to the speech controlled. magnetizing current at the transmitting station and is, therefore, interpretable in the telephone In both Figs. 2 and 3 it is desirable that the inductance should be a linear function of the unidirectional current traversing it, or preferably that the resonance frequency of the tuned circuit 1. should be a linear, function of the unidirectional magnetizing current.

The operation of the simplified circuit of Fig. 3 is attended with an increased tendency toward distortion as a result of any casual amplitude systems to insert current limiters at either the transmitting or'receiving station or both.

The circuit of Fig. 4 corresponds in the tuning of the path 39, id and in the function of the magnetizing winding 4! to the circuit of Fig. 3. Circuits of this general type in which electricaltuningiis a function of variable ferromagnetic'inductances are subject to distortions occasioned by non-linearity in the relationship existing between ithe path 39, 50 it serves through winding 33 to likewise vary the resonance frequency of path 44, 45. The path 44, 45 is connected in shunt across a local circuit including a source 46 of constantfrequencyoscillations, a series resistance tances 25 and an amplitude demodulator 48 of the usual type to the input circuit of which a receiver 59 is connected through transformer 55. The-lccal source 46-- is constructed and adjusted tov produce oscillations of the normalresonance frequency of the path.M,- '15. Consequently, during non-modulating intervals the path 44, 45 serves as a short circuit, preventing oscillations from the source 46 from reaching the detector 48.

As. the resonance frequency of the path it, 3.5v varies in response to the-variable magnetizing currents its impedance likewise varies. with the result that .there'are'transmitted from the source 46 to the demodulator d8 oscillations having amplitude--modulations which may be readily de modulated by the demodulator. Sincetheopejration of the magnetizing. circuit 43 on-the' path M, 55 is the converse ofthe operation of the path 39, lil-v upon the magnetizing circuit atll, it follows that thedistortion introduced in the original conversion is counteracted. Accordingly, the

amplitude of the modulated waves impressed upon-thedemodulator 68' will vary substantiallyin accordance with the frequency of the incoming carrier waves which inturn varies in"accordance-with the amplitude of the modulating currents at the remote radio transmitter. The output of detector 48 will, therefore; transmit to receiverdB demodulated currents which correspond in frequency and wave form to the-original modulating signals.

The receiving system of Fig. involves an alternative expedient for counteracting distortion which may occur in consequence of energy trans.- lation by the-control. of a ferromagnetic induct.-

For that area-c an'ce. The. elementsasimilarly designatedcorre spond to those of Fig. 4. In series withnthe corrtrol winding 4! in the'output circuit of theid'etectorl tube isa device :5!" comprising a non-linear resistance whichlmay'beof the well-known copper-oxide varistorytype: The :device is preferably 1 so designedas; to have. a; characteristic. such thatthetvoltage: across it. isthe samefunction-of the current through itas: the-frequency of the reso-' nant circuit isiofathe current through the ferromagnetic coil; Since thesam'ecurrentatraverses the ferromagnetic coil 41- andv the varistor 5| thevoltageiacross the-varistor 5twi1l'vary'in accordancerwiththe frequency of the resonant circuit. Inasmuch as. the; frequency of theresonant cir cuitchangesfrom instanttoinstant to follow the carrier frequency of the incoming received waves,-v theintensity of the electromotive force 1 across the 'v-aristor 5 l' w-ill likewise follow-the carrier-frequency; of: the incoming; waves and there will. be: supplied to the, transformer: 50 and. the receiving instrument is accurrentgWhich-is a--.replica::.of the modulating: wave atthe remote transmitting sta' tion;

At the transmitting; station; the-samev problems as to non-linearittl' arise in connection withwthe use or. variable; frequency circuits-including ferro' magnetic inductancescontrolled. by the.-mo.dulatingg current. One method. of reducing such: nonlinear-ity, in aicirlcuit like that of Rigel. istoutilize I as theresistance element E-Bpf'thatcircuit anonlinear resistance so designed asto compensate :for non-linearity oft the..-.tuned. circuit: characteristic with respect. tomodulating current. Thenon-3 linear. resistance Hi. shouldbe-so designed that" the resonant frequency-of..thetunedcircuit 'Il is a linear function of the modulatingi-electromotive force. impressed on the magnetizing circuit at the; terminals of the secondary winding: of trans-- former 22-. 7

Antalternative circuitfor eliminating distor-- tion'at the transmitter .is-that ofiE'ig; 6. Inthis circuit corresponding-elements are designatedas il1=Flg..1-. The modulating input circuit isconnected through a series=high resistance lfi-tothe input terminals of-..an amplitude modulator 53 with. which is'associated-a carrier frequency os-; cil1ator54. Although the. precise frequency of the oscillations.- produced by oscillator 54 a is immaterial it is preferably of the same order as the frequency, of-the principal oscillator Si. It is desir-able thatoscillator 54 be highly stablewith respect to. both. amplitude. and frequency soas; not= to introduce additional distortion for which the correcting circuit. must. correct Modulated 55 oscillations delivered. at the output terminals of modulator. 53 are-impressed upon the input terthe" feedback loop including the series resistance and the shunt inductance 57 so" that the out put of .the demodulator 55 is. substantiallyr free" from distortionw In" the same manner asin the.

, 53;..demodulator- 55 or the connecting circuit of circuit of i Fig. 1, the speech input circuit 1mpresses a speech input electromctive force upon a path leading to reactance control winding [4' associated with the ferro-magnetic inductance l3. The path of the control winding l4 also includes in series with the control winding a ferromagnetic reactance 58 so associated with the shunt inductance 51 as-to be controlled. thereby. The cores of the ferromagnetic reactances 58 and I3 are constructed of the same material and the reactances of both are therefore variable with current amplitude. The variable amplitude audio frequency current in the output or space current circuit of modulator 53 induces in winding 58 an audio frequency electromctive force which is distorted by the action of the variable reactance 58. The distorted electromctive force induced in winding 58 is superposed upom an undistorted speech electromctive force'derived from points 59, 60, and the resulting distorted speech current traverses control winding I4. The variable reactances are preferably so designed that the distortion in the speech current applied to the control winding is of such character and magnitude as just to counteract the distortion which occurs in inductance l3. More precisely stated, the current traversing winding I4 is so predistorted that the're'so-nantfrequency of the circuit I! will vary directly and proportionately in accordance with the undistorted speech electromctive force between points 59 and 60. The control circuit including windings l4 and 58 is designed to transmit audio frequency currents and the transfer of high frequency is precluded T by the character of winding 58 which has such high distributed capacity as to effectively shortcircuit any carrier frequency electromctive force that might otherwise be induced therein.

Fig. 7 shows the circuit of an alternative type of frequency modulation transmitter which also employs a reverse feedback path .to maintain linearity between the outgoingcarrier frequency and the amplitude of the modulating waves. The circuit may be considered as a modification of Fig. l with a reverse feedback circuit connected between the input-terminals 62, '63 of banchpass filter I! and the speech input terminals 64 and 65 in the modulating path. Included in the reverse feedback path is a frequency demodulating element 66 which includes sufficient incidental or additional amplification to enable the distortion correcting action of reverse feedback to be attained. If aspeech or other modulating electromctive force is impressed between points 64 and 65 the'carrier frequency of the modulated carrier wave as presented at output terminals 62 and 63 should faithfully follow the.

intensity of the modulating electromctive force. Consequently, frequency-demodulating device 66 which may, for example, be of the types disclosed in Figs. 2, 4 or will yield an undistorted speech frequeneyelectromotive force. If, however, there is any tendency for distortion to occur in the modulation process because of the characteristics of the ferromagnetic inductance or of the tunedcircuit there will be a corresponding tendency for the speech frequency current in the output path of demodulator 66 to exhibit a corresponding distortion. Any such tendency is counteracted by the operation of the negative feedback path which insures that such a modulating or control current is applied to control winding M as will overcome the distorting tendency.

Fig. 8 presents a frequency modulated wave receiving system in which two detectors are connected like detectors 32 and 33 of Fig. 2 with their input terminals in push-pull relation to the receiving circuit and their output terminals inopposition. Any demodulated components resulting from amplitude modulations of the incoming waves passed by band-pass filter 24 will accordingly be balanced out in the output circuit. A local oscillator comprising an electron discharge device '69 and its associated circuits is connected to the'common input path of the two detectors by a transformer 10 and serves to impress on the input path oscillations of the normal unmodulated carrier frequency to which the tuned loop H is resonant, the impressed electromotive force being in phase quadrature to the incoming wave unmodulated carrier frequency electromctive force applied to the same path. The tuned loop includes a ferromagnetic reactance winding 12 with which is associated a control winding I3 connected to the output circuit of the two detectors in the same manner as is control winding 30 of Fig. 2. 'A biasing source 14 corresponds in its function to source 3! of Fig. 2. In operation, the local source tends to remain in frequency agreement and in phase quadrature with the central frequency component of the incoming frequency modulated waves. Any departure from that condition gives rise to an increase in the space current of one detector and a decrease in the space current of the other to produce a control electromctive force which is applied to the winding 13 in such direction asto cause a compensating change in the reactance of element 12 so that the conditions of frequency agreement and phase quadrature are maintained,

Whatis claimed is:

l. A receiving system for frequency modulated carrier waves comprising a tuned circuit, means for impressing received frequency modulated waves upon the tuned circuit, means connected to the tuned circuit for producing a lowfrequency current corresponding in its amplitude to the carrier frequency potential set up across the tuned circuit by the received waves impressed thereupon, and means controlled by the low frequency current to shift the tuning of the tuned circuit in such direction as to tend to reduce the carrier frequency potential thereacross. l 2. A receiving system for frequency modulate carrier waves comprising a circuit tuned to a predetermined frequency having a definite relation with respect to the frequency of the unmodulated carrier waves, means for impressing received frequency modulated waves upon the tuned circuit, means for deriving from the tuned circuit an electromotive force proportional to the 1 of the received waves from which. no component has been suppressed to obtain demodulated waves which vary in amplitude as the frequency of the incoming waves departs from the normal carrier frequency and utilizing the demodulated waves to vary the frequency of the. component which is being suppressed.

4. A receiver for frequency modulated waves comprising an input circuit having a shunt path tuned to the normal unmodulated'carrier'frequency and-inciudinga variable reactance-e1e- 'ment; a detector having input" terminals conto receive the-detected-waves and" means connected to the output circuit coupled to the variable reactan'ce-element and controlled byth e detected waves therein" for 'varying the reactance of the variable reactance elementto cause-the tuning of the-shunt path to"t'en'd to follow the varying frequency of the received frequency mod ulated waves.

5. Afrequency modulation system comprising a pair of dernodulating devices having input circuits ando'utput circuits, the input circuits having a'portion in common-and other portions connected' at one terminal, the output circuits being connected in series opposition, means for applying a received modulated carrier wave to the common portion of the input' circuits, and means for applying a portion of" the same received car-= rier wave to the oth nportionsof the input circuit in opposite phase with respectto the two demodulating devicesand with an intensity which is zero at the unmodulated carrier frequency and which is generally proportional to the frequency departure of the carrier Wave from the normal 6, A receiving system for frequency modulated carrier waves comprising a tuned circuit, means for impressing the receivedlmodulated waves upon the tuned circuit, demodulating means connected to the tuned circuit for producing a low frequency current corresponding in its amplitude to the carrier frequency potential'set up'across the tuned circuit by the received waves impressed thereupon, andmeans for causing the low frequency current'to shift the tuning of the tuned circuit in such direction as to tend to reduce the carrier frequency potential therea'cross- '7. In a wave transmission system, an input circuit upon which received energy'ma'y b'e'im pressed a series tuned circu'it connected: in shunt 7 across the input-circuit to"sliort'circuit received currents of the frequency for which the series; circuit is resonant, an electron discharge device having an output circuit and an input: circuit,

- means couplingthe tuned circuit'tothe input 'ci cuit, the timed circuit comprisingan' inductance having a ferromagnetic core, and means coupled to theoutput circuitto derive control currents therefrom andto impress a corresponding'magnetom'oti ve forc'e upon -the core to cause the resonanceirequencyof-the' tuned circuit to vary in accordance with the amplitude of the control currents whereby" the frequency of the compcnentof the received waves short-circuited from theinput circuit bythe tuned circuit varies in ac -L cor-dance: with the amplitude/of the control curj rents? 8311i combination, a receiving: circuit upon which received Waves are impressed at tuned patn comprising an inductance element having'a ferromagnetic core in series-with a" capacitance, the tuned" path being connectedin shunt across the receivingcircuit to divert therefrom. and supprcss osciliationsof the resonance frequency. of

thetune'crpath; a control circuit including a controiwinding electromagnetically coupled with the of th'e tuned' path varies as theelectromotive forces applied to -the: control circuit vary, and means responsive to a departure from a linearv relationship" betweenthe resonance" frequency and the impressed. control circuit electromotive forces to: so afiect the current in the control winding oi the inductance element as; to tend to restore therelationof linearity between the resonance frequency and the electromotive force impressed on the control circuit whereby the frequency of" the; oscillation component suppressed from thereceived'waves is substantially propor tional to the" amplitude of the control winding current! g 9.- In'ia? frequency modulation system," a receiving circuit over w'hi'clrhighfretulencywaves are received; a serie-situned path connected in shunt to the 're'ceiving'circuit to short-circuit therefrom waves oi the're'sonant frequency of the tuned varied and the-resonance frequency of the tuned circuit is' caused" to vary" as the electromotive force 'applied toth'e' control circuit varies, and: means in circuit with the control winding vto so affect the current in theqwinding as to tendto maintain a iixiear'relationship'between the reso- 1 nancefre'quency of the tuned circuit andthe' intensity of the *eie'ctromotive' force impressed upon thecontroFci-rcuit'. i

i HARRY. NYQUIST.

inductance element to impress magnetomotive forces thereon whereby the resonance frequency 

